1. The Field of the Invention
The present invention relates generally to optical transmitters and receivers. More specifically, the present invention relates to mechanisms for automatically determining the extinction ratio of an optical signal using duty cycle modulation of the optical signal.
2. Background and Relevant Art
Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The electro-optic transducer emits light when current is passed through it, the intensity of the emitted light being a function of the current magnitude. Data reception is generally implemented by way of an optical receiver (also referred to as an optoelectronic transducer), an example of which is a photodiode. The optoelectronic transducer receives light and generates a current, the magnitude of the generated current being a function of the intensity of the received light.
Information is conveyed over an optical fiber by transmitting different optical intensities on the fiber. Generally speaking, a relatively high optical power is transmitted onto the optical fiber to assert a logical high onto the fiber. A relatively low optical power is transmitted onto the optical fiber to assert a logical low. The high optical power is obtained by asserting a higher current to the laser. The low optical power is obtained by asserting a lower current to the laser. The laser is not turned off because it takes significant time to saturate a laser to the point where it begins to lase if starting from a laser that is off. In fact, if the current through the laser were to drop below a certain threshold current, it can take much longer to transition to the high optical intensity. In high data rate applications, this could cause significant jitter and possible degradation of the signal. Accordingly, even the low current that enables the low optical intensity should be kept above the threshold current of the laser. If this constraint is met, the laser can transition quickly from the high optical level to the low optical level, and vice versa.
An additional constraint to the high and low optical levels is referred to as the “extinction ratio”. The extinction ratio is the ratio of the high optical power level to the low optical power level. The optical high and low power levels are obtained by modulating the current between the higher and lower currents. Typical extinction ratio values range from perhaps 6 dB to 12 dB, with higher levels generally being better but more costly in terms of power requirements.
However, maintaining of a proper extinction ratio is more complex than simply statically determining an appropriate high optical level and an appropriate low optical level, and keeping with that level. Varying temperatures have a profound effect upon the extinction ratio. FIG. 3 illustrates approximate laser current versus optical power curves for several different temperatures including 0, 25 and 70 degrees Celsius. The threshold temperature for 0, 25 and 70 degrees Celsius are illustrated as TTH0, TTH25 and TTH70. The difference in the curves for varying temperatures is exaggerated to illustrate the principles of temperature dependency in the curve. Each laser will have slightly different curves shapes and temperatures dependencies. However, regardless of the laser, the laser tends not to emit significant optical power if the supplied current is below the threshold current. In addition, for all lasers, as temperature rises, threshold current increases and the slope of the curve in the linear region above the threshold current (i.e., the slope efficiency) reduces.
FIG. 3 also shows the low optical level PLOW25 for 25 degrees Celsius and the corresponding current ILOW25 needed to attain that low power level at 25 degrees Celsius, and a high optical level PHIGH25 also for 25 degrees Celsius and the corresponding current IHIGH25 needed to attain that high power level at 25 degrees Celsius.
As temperature rises, the threshold current needed for the laser to transmit any significant degree of optical power rises. In addition, the slope of the curve in the linear region above the threshold current becomes less steep. This means that if the temperature were to fall or rise, the optical power emitted by the laser given a constant current will also change. Accordingly, in order to maintain a proper extinction ratio, the extinction ratio is periodically checked and adjusted if needed. This allows the optical transmitter or transceiver to operate under wide-ranging temperature conditions without introducing inordinate amounts of jitter into the transmitted signal, and while maintaining a roughly constant extinction ratio.
The subject matter, claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.